机械精度设计与检测技术 (13).pdf
STRAIGHTNESS MEASUREMENTSECTION 4.2Straightness MeasurementStraightness Measurement is a very important technique in the field of mechanical engineering.The straightness measurement is employed primarilyin monitoring the line elements in cylindrical，conicaland flat surface.It can also be used to control straightness deviations ofaxes in cylindrical and conical.Straight elementsCylindrical partsConical partsFlat surfaceStraight elementsStraight elementsStraightness MeasurementFor checking the straightness of any surface,we candirectly compare the surface with geometrical idealstraight reference line.The ideal straight line may be established by a straightedge,a wire with tight tension,a measuring plat,anoptical flat,a laser beam of autocollimator etc.Straight edgeAutocollimatorMethod(1):Direct Comparison Method-Straight EdgeMethod(1):Direct Comparison Method-Straight EdgeThe most convenient way to measure straightness is toplace a straight edge against the workpiece surface andthe gap between them is viewed against the white light.By observing the size of the small gap,the straightnessdeviation can be measured.Straight edgeGapSurfaceStraightness is measured by observing the color of lightby diffraction while passing through the small gap.For example,if the color of light is red,it indicates a gapof 1.25 to 1.75m.If it is blue,a gap is 0.8m,If it iswhite,a gap is 22.5m.If a gap is large,a feeler gauge is used to measure thegap.Straight edgeGapFeeler gaugeMethod(1):Direct Comparison Method-Straight EdgeMethod(2):Direct Comparison Method-Measuring PlateStraightnessdeviationcanbemeasuredbyadialindicator placing on measuring flat and moving along onedirection of the workpiece surface.Straightness deviation is the maximum difference of a dialindicator reading in several tests.Workpiece measuredFixed supportAdjustable supportDial indicatorPlateMovementAccording to the drawing,the workpiece is measured in 8points and dial indicator readings are list below.Measuring number Indicator reading001-22+73+134+265+206-77+7012377200.0320.015/200456Workpiece measuredPlateFix supportAdjustable supportDial indicatorTwo end points line assessment:Straightness deviation:f1=AC=35 32 (unqualified)Local maximum straightness deviation:f2=PE=20.5 15 (unqualified)LengthIndicator reading(m)1234567-100+10+20+30ACPEf2=20.5mf1=35mMethod(2):Direct Comparison Method-Measuring PlateAccording to the drawing,the workpiece is measured in 8points and dial indicator readings are list below.Measuring number Indicator reading001-22+73+134+265+206-77+7012377200.0320.015/200456Workpiece measuredPlateFix supportAdjustable supportDial indicatorMinimum zone assessment:Straightness deviation:f1=AC=31 15 (unqualified)LengthIndicator reading(m)1234567-100+10+20+30AMPEf2=20.5mf1=31mMethod(2):Direct Comparison Method-Measuring PlateMethod(3):Indirect Measurement MethodIndirect measurement method can also be used,such assquare frame spirit level method.Square frame spirit level has a bubble tube on bottombase.At slight inclinations,the bubble in tube travels anddegree of inclination can be measured by reading themark on the tube.Main bubbleHorizontal bubbleVertical baseBottom baseSquare Frame Spirit levelUsing frame spirit level,the relative angular positions ofnumberofadjacentsectionsofthesurfacearemeasured,and measured values are drawn into a graph.Then,straight deviation is calculated by the graphicalevaluation.Measurement LengthL=300300300300Measuring value：2、1、2、3Mapping to solve:Two end points line assessment:f 1=0.018minimum zone assessment:f 2=0.0153006009000XYStraight deviationf2=0.0153006009000XYStraight deviationf1=0.018Method(3):Indirect Measurement Method
